The most important development in electrolysis electrodes in recent years has been the advent of so-called dimensionally stable anodes following the teachings of U.S. Pat. Nos. 3,771,385 and 3,632,498. The most successful electrocatalytic coatings for such anodes have been those consisting of a mixed oxide of a platinum-group metal and a valve metal forming a mixed crystal or solid solution in which the precious metal oxide is stabilized without detriment to its catalytic characteristics. These coatings, in particular ruthenium-titanium oxide coatings, have been especially successful in chlorine production in mercury cells, diaphragm cells and, more recently, in membrane cells.
The above patents and many others have described multicomponent electrode coatings in which thermodecomposable compounds of the components are mixed in a solution which is repeatedly applied to the electrode substrate, dried and converted to the multicomponent coating by baking. In this way, it is for example possible to provide electrodes with an outstanding lifetime per gram of precious metal employed, as described in U.S. Pat. No. 3,948,751, or electrodes with ion-selective properties for halogen evolution and oxygen inhibition as described in U.S. Pat. No. 4,272,354.
Multilayer electrode coatings produced by building up alternate layers of different materials have also been proposed. For instance, U.S. Pat. No. 3,773,554 describes alternate layers of ruthenium oxide and titanium oxide and U.S. Pat. No. 3,869,312 describes alternate layers of a ruthenium-titanium mixed oxide material and of titanium oxide.
It has also been proposed to anchor or embed an electrochemically active material in an inert layer typically consisting of a layer of titanium oxide on a titanium substrate. Early proposals were to form this layer by heating a titanium substrate in air or by anodic oxidation of a titanium substrate as described in U.S. Pat. No. 3,234,110. A later proposal was to electrocoat Ti with titanium oxide from a solution containing Ti.sup.4+ ions, see U.S. Pat. No. 3,773,555 and 4,039,400. These proposals and their drawbacks are discussed in U.S. Pat. No. 4,140,813 which set out to improve the resistance of the electrode coatings to contact with mercury amalgam by plasma spraying a layer of titanium oxide in the pores of which an active electrode material is anchored.
U.S. Pat. No. 4,223,049 discloses an electrode with a conductive base having a coating of titanium oxide or tin oxide into which ruthenium oxide is superficially mixed by immersion/washing/baking without forming a separate outer layer of ruthenium oxide.
Various proposals have also been made in which an outer layer of electrochemically active material is deposited on a sub-layer of an active material which serves primarily as a conductive intermediate to protect the substrate. For example, U.K. Pat. No. 1,344,540 provided an electrodeposited layer of cobalt oxide or lead oxide under a ruthenium-titanium oxide or similar active outer layer. Various tin oxide based sub-layers are disclosed in U.S. Pat. No. 4,272,354, 3,882,002 and 3,950,240, once again a coated with the same type of active outer layer. U.S. Pat. No. 4,331,528 made an important improvement in this area by developing a preformed barrier layer formed as a surface oxide film integral with and grown up from the valve metal substrate with simultaneous incorporation of a small quantity of rhodium or iridium as metal or oxide in the surface oxide film, the active coating being subsequently deposited on top.
Along similar lines, Japanese Patent publication 028262/78 provided an undercoating of an oxide of ruthenium, tin, iridium or rhodium on a valve metal substrate, and an active outer coating of palladium oxide or a mixture of palladium oxide and ruthenium oxide. In Japanese Patent publication 115282/76, a spinel-type underlayer consisting preponderantly of Fe.sub.2 O.sub.3 with other non-precious oxides was coated with a top-layer of precious metal oxides.
U.S. Pat. No. 4,203 810 has proposed to electroplate a relatively thick layer of a platinum group metal onto an undercoat of a chemideposited platinum-group metal or oxide. The converse arrangement is described in published European patent application No. 0,090,425, in which an oxide of ruthenium, palladium or iridium is chemideposited into a porous layer of platinum electroplated onto an electrically-conductive substrate.
Other proposals for intermediate layers have included an underlayer of ruthenium, rhodium or palladium oxide to which an outer layer of a preformed spinel was attached by means of a binder, see U.K. Pat. No. 1,346,369 and a platinum-iridium undercoat topcoated with a composite containing lead, ruthenium, tantalum, platinum, iridium and oxygen, see Published PCT Patent Application W083/03265.
The prior art discussed above concerns coating formulations intended for the production of new electrodes. It is also known to renovate previously-used dimensionally stable anodes by cleaning the old coating and applying on top a new coating of similar composition, see U.S. Pat. No. 3,684,543. Recently, this so-called top coating procedure has been improved by an activation of the old coating prior to application of the new outer electrocatalytic coating, as described in U.S. Pat. No. 4,446,245. In this case, the activated old coating serves as a base for the new coating. Thus, the teaching of this patent is confined solely to the recoating of previously-used electrodes.
The above-mentioned electrocatalysts are generally coated onto a massive substrate such as a sheet of valve metal, one common configuration being an expanded mesh. Other arrangements are however possible. For example, the electrocatalyst can be particulate or can be supported on particles of suitable material such as a valve metal and these particles may then be applied to a conductive lead substrate (see U.S. Pat. No. 4,425,217) or may be incorporated in a narrow gap electrolysis cell e.g. by bonding to a membrane as disclosed in European Patent Application No. 0,081,251, or they may be used in a fluidized bed electrochemical cell (see U.S. Pat. No. 4,206,020). Other substrate configurations include wires, tubes, perforated plates, reticulated structures and so forth.
Electrodes with catalytic coatings of the types described above may be used in various electrolytic processes. Typically they are used as anodes in chlor-alkali cells or as oxygen evolving anodes e.g. in metal electrowinning processes. Their use as cathodes in various processes has elso been proposed, e.g. for the production of chlorine dioxide, as disclosed in European Patent Application No.0 065 819. The latter patent application also proposed the same materials as heterogeneous catalysts for the non-electrochemical production of chlorine dioxide. Typical catalysts for this application included codeposited oxides of ruthenium/rhodium, ruthenium/rhodium/palladium and ruthenium/palladium usually codeposited with a matrix of titanium dioxide. The catalysts were usually deposited on a titanium substrate but other supports such as alumina were also proposed.
European Patent Publication No. 0099866 describes a catalyst for the oxygen evolution reaction in water electrolysis. This catalyst comprises a host matrix of a transition element namely cobalt, nickel or manganese which incorporates one or more modifier elements deposited for example by vacuum sputtering and then subjected to a heat treatment or an electrochemical treatment. Improved activity is claimed in relation to a nickel anode.
It is also known from U.K. Pat. No. 1,531,373 to place, in the anode compartment of a diaphragm cell, a non-polarized titanium mesh or a polymer lattice coated with a catalytic material such as ruthenium-titanium oxide which functions to catalyze the decomposition of hypochlorite ions.
Thus, broadly speaking, from the prior art discussed above it is known to have a porous high surface area electroconductive catalytic material comprising at least one platinum group metal and/or at least one platinum group metal oxide which is applied to a support, advantageously a porous pre-formed matrix e.g. of titanium oxide. Also, broadly known from the prior art is a porous high surface area electroconductive catalytic material comprising a porous preformed catalytic matrix supporting a subsequently-applied additional catalyst.
In many standard applications such as electrode coatings for chlorine production in diaphragm cells and mercury cells, the known catalytic materials have proven to be outstanding in their performance and cost effectiveness. However, for some applications it still remains desirable to improve the performance without this being offset by a prohibitive cost due either to a high cost of the catalyst or a high production cost or a combination of these.
For example, it would be desirable to provide an economical anode coating with enhanced resistance to caustic for use in membrane cells. Also, there is a need for an economical anode coating with high chlorine selectivity (i.e. selective inhibition of oxygen evolution) for use in dilute chloride solutions, in chlorate production cells or seawater electrolysis. There is also a need for an anode coating with low oxygen overpotential and long life in sulphuric acid for metal electrowinning from sulphate solutions. And in some mercury cell plants where operating conditions are particularly severe it would be desirable to improve the resistance of the anode coatings to contact with amalgam.